Two indices of autonomic maturation in newborns are measurements of heart rate variability (HRV) and changes in heart rate (HR) in response to tilt. In the current study we have investigated the relationship between these assays in 48 healthy, full-term infants between 18 and 36 hr of age. While sleeping, each infant was tilted 30° four times; two head-up, two head-down. The duration of each tilt was 30 sec, head-up and head-down positions were maintained for 60 sec, and there was a 60 sec inter-tilt interval. R-R intervals were collected continuously and were converted to instantaneous HR at 10 Hz. HR and HRV (SD of HR) were computed every 10 sec. The average change in HR to head-up tilts was +2.8 (±2.5) bpm and to head-down was -3.0(±2.5) bpm. The difference between these changes (+5.8 bpm) was taken as the overall HR response to tilt (HRT) and was significantly different from zero (t=8.81, p<.001). The average HRV for all 10 sec periods prior to tilt was 3.8 (±1.5) bpm. The correlation between HRT and HRV was +.62(p<.001).

These results confirm prior studies showing a functioning baroreceptor reflex in newborns. Importantly, the data also show that differences in levels of autonomic activity account for much of the individual variation in the magnitude of this reflex response.

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